This invention relates to outdoor intrusion detection using microwave transmissions, and more particularly to a Doppler Radar sensor using two frequencies to derive range information, which is used to optimize the detection process to discriminate against nuisance alarms and detect human intruders.
The detection of human intruders as they cross outdoor perimeters plays a significant role in the prevention of crime. Intrusion detection sensors based on Doppler Radar are in common use today. For example, the assignee produces a number of products based on U.S. Pat. No. 4,697,184 that are used at high security sites around the world.
The major challenge in developing a Doppler Radar for intrusion detection is to detect human targets at ranges up to 400 feet while not detecting very small moving objects such as rain drops near the radar antenna or on the antenna radome. The fact that the power reflected from a moving target decreases as the fourth power of the range to the target makes the reflection from very small objects in proximity to the radar comparable to much larger targets at a long range. There are a number of patents relating to means to cope with this very challenging design problem.
It has been recognized for some time that a Doppler Radar using two frequencies can be used to locate and determine the direction of a moving target. U.S. Pat. No. 3,766,554 issued Oct. 16, 1973 describes a two frequency CW Doppler radar using the phase relationship between the two Doppler responses to create a range cutoff. U.S. Pat. No. 3,832,709 issued Aug. 27, 1974 describes a Doppler radar that alternately transmits on two frequencies and uses circuitry to recreate the two Doppler Responses. In these patents the phase relationship between the two Doppler responses is used to locate the target.
U.S. Pat. No. 4,697,184 issued Feb. 9, 1984 and assigned to the present assignee describes a two frequency Doppler Radar for the detection of intruders. The two frequencies are transmitted alternately and circuitry is used to take a narrow sample of the received Doppler response signal corresponding to each frequency at a fixed time delay from the onset of the transmitted pulse. This timing circuit provides an adjustable well-defined range cutoff while preserving the relatively narrow bandwidth required to meet FFC regulations. The two frequencies are selected so that the Doppler responses are in-phase for targets at the transceiver and are in quadrature-phase at the maximum range. The peak difference between the two Doppler responses is compared to a threshold to detect the presence of an intruder. By adjusting the relative gain of the two Doppler channels, a null is created for targets in proximity to the radome of the transceiver. This minimizes the number of nuisance alarms caused by raindrops on the radome. The installer defined range cutoff and suppression of responses due to raindrops on the radome has proven to be very beneficial to users of these sensors. Despite these benefits there is an ongoing drive to further reduce the number nuisance alarms.
In the sensor based on U.S. Pat. No. 4,697,184 the difference in amplitude of the two Doppler responses is used as the measure of the target amplitude. It can be shown that this provides gain compensation, which follows a sine wave from zero to ninety degrees. The zero is at the radome and the ninety degrees is at the maximum range. While the amplitude of this compensation is the same for the quadrature-phase compensation used in the present invention, it does not provide information as to the axial direction of the target motion or range of the target as does the present invention. The ability to tell target direction provides discrimination against periodic target motion such as due to blowing grass or shrubs that is not possible if one uses only the difference between the Doppler responses.
The objective of the present invention is to use digital signal processing to improve upon the performance of the Doppler Radar described in U.S. Pat. No. 4,697,184. The sensor described in this patent is limited by having only one threshold and one integration time constant to process the very wide range in amplitude response. Specifically, the improvements herein relate to further reducing the nuisance and false alarms rate by using the Doppler phase information to determine the range to the target and optimal response integration and threshold level as a function of range.
The present invention provides microwave Doppler radar detection of intruders in an outdoor environment. A directional antenna is used to transmit pulses of RF energy and to receive signals reflected from targets moving in the detection zone defined by the electromagnetic field pattern of the antenna. The pulses of RF energy alternate in frequency between ƒA and ƒB. An intruder moving in the detection zone creates a Doppler response at each of the two frequencies which when processed provides range and direction of travel information. This range information is used to optimize the signal integration and to apply a range dependent threshold to minimize the nuisance alarm rate.
The radar is designed to detect intruders moving in a detection zone from a minimum to a maximum range. The start of the detection zone is usually determined by the distance at which the antenna field has sufficient vertical beam width to detect a crawling intruder and an intruder attempting to jump over the detection field. The end of the detection zone is determined by the range cutoff setting. The installer defined range cutoff is used to avoid nuisance alarms from objects moving beyond the detection zone.
The time delay between the transmission of the pulse and the receipt of the pulse reflected from the target is directly proportional to the distance between the antenna and the target as determined by the velocity of free space. The transmitted pulse length is selected to be longer than the time delay relating to a target at the maximum range cutoff setting. This relatively long transmit pulse has a sufficiently narrow bandwidth to meet radio regulatory criteria.
A target moving in the detection zone reflects a small percentage of the transmitted pulse back to the receiver. This received signal is detected and a Doppler response is generated. This Doppler response is amplified in a logarithmic amplifier in order to compress the dynamic range of the Doppler response with distance from the transceiver. The number of cycles of Doppler response per unit distance traveled by the target along the axis of the antenna is directly proportional to frequency. Hence the phase difference between the Doppler response at ƒA and ƒB is proportional to the range of the target. The difference between ƒA and ƒB is selected so that the phase angle at the maximum range is ninety degrees. This means that the two Doppler responses are in-phase when the target is at the antenna and in-quadrature phase when the target is at the maximum range.
The Doppler response to each transmitted pulse is digitized during a very narrow sample window interval. The time delay between the onset of the transmit pulse and the sample window is determined by the range cutoff setting. Hence if the target response arrives after the sample window interval, it is not detected. This provides a very precise range cutoff.
The digital signal processing can be explained in terms of a Lissajous figure. If one were to apply ƒA and ƒB Doppler signals to the X and Y input to an oscilloscope, the Doppler signals would create an elliptical Lissajous figure. The ratio of the major and minor axis of the ellipse is a measure of the target range. The direction and speed with which the ellipse is traced are measures of the direction and velocity towards or away from the transceiver. When the target approaches the transceiver, the elliptical Lissajous figure approaches a line at 45 degrees in quadrants 1 and 3. At mid range, the elliptical Lissajous figure approaches a circle. As the target approaches the maximum range, the elliptical Lissajous figure approaches a line at 135 degrees in quadrants 2 and 4.
The two Doppler responses are digitized. Each pair of ƒA and ƒB samples represents a point in tracing out the Lissajous figure. Successive sample pairs effectively trace out the Lissajous figure. Digital signal processing is effectively used to measure the dimensions of the Lissajous figure, the number of sample pairs per revolution of the Lissajous figure and the direction that the Lissajous figure is traced.
The Root of the Mean of the Square (RMS) magnitude of the Doppler responses while in quadrants 1 and 3 is called xe2x80x9cZExe2x80x9d, the amplitude of the xe2x80x9cEvenxe2x80x9d response. The RMS of the Doppler response while in quadrants 2 and 4 is called xe2x80x9cZOxe2x80x9d, the amplitude of the xe2x80x9cOddxe2x80x9d response. In the present invention the sum, ZO+ZE/4, is used as a measure of the amplitude of the target response. This was chosen to provide nearly linear response over the target range of interest with the logarithmic amplifier used.
There are two measures of target range. The ratio of ZE to ZO is a measure of the range of the target from the transceiver. Secondly, the ratio of the number of sample pairs in the even quadrants minus those in the odd quadrants is a measure of the range of the target from the transceiver. The first approach is used in proximity to the transceiver and the second method is used at the further ranges. This range information is used to generate a number of range bins.
The width of the detection zone is largely determined by the beam width of the antenna pattern. In general this means that the detection zone widens with range and an intruder crossing through the detection zone at a constant velocity will take longer to get through the detection zone at further ranges. This means that increasing the period of the integration of the target response for each range bin as the target range increases can optimize Signal to Noise Ratio (SNR) of the response.
The design of the antenna is a major factor in determining the shape of the detection zone. Often the radar is used to detect intruders between chain link fences around the perimeter of a high security site. In general one wants to keep the beam width as narrow as possible so as to minimize unwanted reflections from the fences as they move in the wind. However in elevation the wider the antenna pattern the better the detection of crawling intruders near the antenna. This has led to the design of a contoured dish reflector that optimizes these conflicting requirements.
Target velocity is an important factor in optimizing the detection routine. A running intruder crosses through the detection zone faster than a crawling target. On the other hand a running target presents a larger radar cross section and a larger target response. Having separate thresholds for a High Speed Channel and a Low Speed Channel for each to the range bins further reduces the nuisance alarm rate.
Further features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention when viewed in conjunction with the accompanying drawings.